Fibers are used for many different applications in a wide variety of industries, such as the commercial aviation, recreation, industrial and transportation industries. Commonly-used fibers for these and other applications include cellulosic fiber (e.g., viscose rayon, cotton, etc.), glass fiber, carbon fiber, and aramid fiber, to name just a few.
In many fiber-containing products, the fibers are present in the form of a composite material (e.g., fiberglass, etc.). A composite material is a heterogeneous combination of two or more constituents that differ in form or composition on a macroscopic scale. While the composite material exhibits characteristics that neither constituent alone possesses, the constituents retain their unique physical and chemical identities within the composite.
Two key constituents of a composite include a reinforcing agent and a resin matrix. In a fiber-based composite, the fibers are the reinforcing agent. The resin matrix keeps the fibers in a desired location and orientation and also serves as a load-transfer medium between fibers within the composite.
Fibers are characterized by certain properties, such as mechanical strength, density, electrical resistivity, thermal conductivity, etc. The fibers “lend” their characteristic properties, in particular their strength-related properties, to the composite. Fibers therefore play an important role in determining a composite's suitability for a given application.
To realize the benefit of fiber properties in a composite, there must be a good interface between the fibers and the matrix. This is achieved through the use of a surface coating, typically referred to as “sizing.” The sizing provides an all important physico-chemical link between fiber and the resin matrix and thus has a significant impact on the mechanical and chemical properties of the composite. The sizing is applied to fibers during their manufacture.
Substantially all conventional sizing has lower interfacial strength than the fibers to which it's applied. As a consequence, the strength of the sizing and its ability to withstand interfacial stress ultimately determines the strength of the overall composite. In other words, using conventional sizing, the resulting composite cannot have a strength that is equal to or greater than that of the fiber.